Fact sheet: High temperature thermal desorption

From: Public Services and Procurement Canada

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Description

High temperature thermal desorption (HTTD) is a process in which excavated contaminated materials are heated in a chamber, thereby volatilizing water, organic contaminants and certain metals. HTTD involves temperatures ranging from 315 °C to 538 °C (600 °F to 1,000 °F), while low temperature thermal desorption (LTTD) involves temperatures ranging from 90 °C to 315 °C (200 °F to 600 °F). Higher temperatures used in HTTD facilitate removal of semi-volatile organic compounds (SVOCs) (<0.01 mm Hg vapour pressure). Higher temperatures, however, may also change the physical properties of the soil, and consequent loss of organic matter may inhibit biological activity.

In contrast to incineration, the operating temperatures and residence times in a thermal desorption system are designed to volatilize selected contaminants without oxidizing them. Thermal desorption treatment is not designed to destroy organic.

Treatment of contaminated soil using a thermal desorption system requires excavation and transportation of the soil to the reactor, where it is heated to a predetermined temperature. Two common thermal desorption reactor designs are the rotary dryer and the thermal screw. Throughput rates can vary from less than 5 to 10 to approximately 50 metric tons per hour depending on the type of soil and treatment unit. Ex situ thermal remediation systems use relatively large amounts of energy to heat the excavated soil. The treated soil can be used on-site as backfill if it meets regulatory requirements, or it may be disposed of off-site.

A gas or vacuum system transports the vaporized water and contaminants to an air emission treatment system, where particulates and contaminants are removed.

Internet links:

Implementation of the technology

HTTD systems may include:

  • Physical and chemical characterization of the soil or sediment;
  • Site preparations (clearing/grubbing/demolition, topsoil stripping and temporary stockpiling);
  • Mobilization of equipment (including installation of thermal desorption system, construction of temporary facilities and site access considerations);
  • Excavation:
    • Excavation dewatering;
    • Slope stability controls;
    • Foundation protection for retained structures (shoring, underpinning, etc.);
  • Soil screening to separate and remove (or crush) oversize materials, if present, before placement in heating units;   
  • Soil drying (required only if the moisture content of the excavated soil exceeds 20 to 25%);
  • Soil heating in a rotary dryer or a thermal crew. Rotary dryers are horizontal or inclined cylinders that can be indirectly or directly fired. In thermal screw units, screw conveyors or hollow augers are used to transport the soil through an enclosed trough. Hot oil or steam circulates through the auger, heating the soil indirectly;
  • Gas collection and treatment through thermal oxidation, condensation, or adsorption (including removal of dust particles);
  • Compliance sampling of treated soil stockpiles;
  • On-site disposal of soil (backfilling excavations or spreading on-site) or off-site disposal;
  • Decommissioning and removal of thermal desorption system and gas collection and treatment systems;
Surface restoration (Planting, paving, etc.)

Captured vapours including water vapour and volatile organic compound (VOC) vapours require treatment for the removal of particulates and contaminants. Particulate removal equipment may be wet scrubbers or fabric filters. Condensation or adsorption (for example through GAC) equipment may be used for contaminant removal. Alternatively, the contaminants in the off-gas may be destroyed in a thermal oxidation system, which can be operated flameless, with a direct flame or as a catalytic oxidation system. A carrier gas or vacuum is used to transport water and VOC vapours to the gas treatment system.

Materials and Storage

The method relies on traditional, commonly available civil/earthworks construction equipment and methods for the excavation component. Commercial and transportable units are available for the treatment component. Depending on the soil throughput rates, units may be mounted on one to five trailers.

  • Sufficient storage space is required to house the thermal treatment system;
  • On-site stores are typically limited to small amounts of fuel and lubricant (daily fuelling of excavators is often from a mobile tank) as well as miscellaneous construction site supplies;
  • Contractors may create temporary stockpiles of contaminated materials pending treatment;
  • To protect from rain and minimize soil moisture content, the soil stockpiles and feed equipment need to be covered.

Waste and Discharges

  • Residuals from thermal desorption include treated off-gas, particulates, filters, catalysts, non-contact combustion gases. Generation of off-gases containing dioxins and furans, and/or halogenated acids can occur during thermal desorption treatment of soil with halogenated compounds. While there are processes that have been designed to minimize generation of dioxin and furans and/or remove these compounds from off-gas through incineration, treatment of halogenated compounds should be approached with caution; 
  • Windblown dust can occur from soil loading, track-out or stockpiles, for example, may also deposit directly on downwind surfaces (stormwater can also be impacted by dust).
  • Solid discharges from thermal desorption include particulates;
  • Spent activated carbon could require periodic off-site transport and regeneration or disposal. If a wet gas scrubber is used, the resulting sludge from particulates in the wastewater stream requires proper disposal;
  • Diverted and collected/treated stormwater are typically passed into the local stormwater system;
  • Thermal desorption produce off-gases that requires monitoring and potentially treatment prior to release to the atmosphere; 
  • In specific cases, naturally occurring radon may also be mobilized.

Recommended analyses for detailed characterization

Physical analysis

  • Soil water content
  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure

Recommended trials for detailed characterization

None.

Other information recommended for detailed characterization

None.


Notes:


Notes: Treatability tests are recommended to determine the efficiency of thermal desorption for removing various contaminants at various temperatures and residence times.

Applications

  • The target contaminants for HTTD are SVOCs, PAHs, PCBs and pesticides. VOCs and fuels may also be treated, but HTTD may be less cost-effective for these contaminants;
  • Volatile metals may be removed by HTTD systems;
  • This process is applicable for the removal of organs from refinery waste, coal tar waste, wood treatment waste, creosote-contaminated soils, hydrocarbon-contaminated soils, synthetic rubber processing waste, pesticides and paint wastes;
  • This ex situ process is mobile and can be transported to the site.

Applications to sites in northern regions

Remote sites are prone to high mobilization and on-site monitoring costs, limited equipment availability and short work windows.

Since it requires large and complex equipment and high energy consumption, thermal desorption is not well adapted for northern and remote environments.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Does not apply
Ex situ
Applies
Biological
Does not exist
Chemical
Does not exist
Control
Does not exist
Dissolved contamination
Does not exist
Free Phase
Does not exist
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Applies

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
Applies
Explosives
Does not apply
Metals
With restrictions
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Does not apply
Pesticides
Applies
Petroleum hydrocarbons
Applies
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
Applies
Polychlorinated biphenyls
Applies

Notes:

Depending on the volume of the soil requiring treatment, the treatment plant may be in operation from weeks to months.

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Does not apply
3 to 5 years
Does not apply
More than 5 years
Does not apply

Long-term considerations (following remediation work)

There are no major long-term considerations related to thermal treatment systems. If the treated soil is used as backfill, minor long-term considerations may include changes to the geotechnical properties of the soil or changes to the organic content of the soil due to the decomposition of soil components during heating.

Secondary by-products and/or metabolites

Control and treatment of air emissions from thermal desorption operations are an extremely important consideration. Generation of off-gases, containing dioxins and furans, and/or halogenated acids can occur during thermal desorption treatment of soil with halogenated compounds. The system should be designed, operated and maintained to prevent the emission of metals, polycyclic aromatic hydrocarbons (PAHs) or dioxins/furans.

Limitations and Undesirable Effects of the Technology

  • Highly abrasive debris in soil can damage the processor unit;
  • Clayey, silty and high humic content soils require increased residence time because of binding of contaminants;
  • Dust and organic matter in the soil increase the difficulty of treating off-gases;
  • The presence of chlorine can affect the volatilization of some metals such as lead;
  • Soil storage piles need to be covered to protect from rain (to minimize water infiltration) and from wind;
  • Leaching of mercury from stockpiled soil into groundwater is of concern;
  • Thermal desorption of mercury-contaminated waste is not recommended;
  • Treated soil may no longer support microbiological activity, which may be of concern if the soil is returned to a previously or partially contaminated site; 
  • Treated soil may exhibit different geochemical properties than untreated soil. If the treated soil is used as backfill, treated soil can thus impact the in-situ geochemical conditions;
  • The physical disruption of excavation is significant; cave-ins, slumping and related damage to nearby structures are possible if geotechnical & civil engineering works are inadequate. Dramatic, although short-lived, changes to site-scale hydrology and hydrogeology are common. Large excavations below the water table typically require either groundwater cut-off walls or extensive pumping, both of which alter flow paths at the site scale;
  • The potential for off-site receptors to be exposed to contaminated vapour is limited, provided that the off-gas treatment system is properly designed and operated. However, care must be taken to match treatment technology with the type of contamination to prevent unintended emissions of toxic compounds. For example, depending on the thermal desorption process and temperature, dioxins and furans, and/or halogenated acids may be generated when treating halogenated contaminants;
  • Handling, for example, fuel vapours at levels near the lower explosive limit (LEL) and/or using supplemental fuel or reactive oxidation catalysts may pose a fire/explosion risk. Designers typically specify special “intrinsically safe” equipment in areas where flammable vapours are handled and incorporate ventilation, alarm, control interlock and fire suppression measures;
  • The limitations to soil excavation in general apply (slope stability considerations and protection, groundwater dewatering, protection of infrastructure, safety measures, etc.).

Complementary technologies that improve treatment effectiveness

  • HTTD is frequently used in combination with incineration, solidification/stabilization or de-chlorination, depending upon site-specific conditions.

Required secondary treatments

  • Control and treatment of air emissions;
  • Dust control system;
  • Treatments such as dewatering, sizing, crushing, blending with sand or removing debris may be necessary prior to thermal desorption;
  • Heavy metals in the contaminated material may produce a treated solid residue that requires stabilization.

Application examples

Application examples are available at these addresses:

Performance

This technology is capable of reducing final contaminant concentrations to below 5 mg/kg for target compounds. HTTD costs in Canada are competitive with landfills or biological treatment (CSMWG, 2005).

The time required to complete cleanup of a “standard” 18,200-metric ton (20,000-ton) site using HTTD is just over 4 months (Reference).

Measures to improve sustainability or promote ecological remediation

  • Use of renewable energy and energy-efficient machinery (such as geothermal, wind or solar energy).
  • Process optimization to reduce wastes and consumables.
  • Schedule optimization for resource sharing and fewer days of mobilization.
  • Assessment of pre-treatment options for feedstock to increase efficiency of thermal treatment system (such as optimum soil moisture content).

References

Author and update

Composed by : Josée Thibodeau, M.Sc, National Research Council

Updated by : Martin Désilets, B.Sc., National Research Council

Updated Date : March 1, 2008

Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée

Updated Date : March 31, 2018

Version:
1.2.1